Lithium silicate glass ceramics are characterized as a rule by very good mechanical properties, which is why they have been used for a long time in the dental field and there primarily for the preparation of dental crowns and small bridges. The known lithium silicate glass ceramics usually contain as main components SiO2, Li2O, Na2O or K2O, and nucleating agents such as P2O5 as well as additional components.
DE 24 51 121 describes lithium disilicate glass ceramics which contain K2O and Al2O3. They are prepared from corresponding nuclei-containing starting glasses which are heated to temperatures of from 850 to 870° C. for the crystallization of lithium disilicate. The purpose for which the glass ceramics are used is not disclosed.
EP 827 941 describes sinterable lithium disilicate glass ceramics for dental purposes, which also contain K2O or Na2O in addition to La2O3. The lithium disilicate crystal phase is produced at a temperature of 850° C.
Lithium disilicate glass ceramics which likewise contain La2O3 as well as K2O are known from EP 916 625. A heat treatment is carried out at 870° C. for the formation of lithium disilicate.
EP 1 505 041 describes lithium silicate glass ceramics containing K2O, which, when lithium metasilicate is present as main crystal phase, can be very satisfactorily machined e.g. by means of CAD/CAM processes, in order to then be converted by further heat treatment at temperatures of from 830 to 850° C. into high-strength lithium disilicate glass ceramics.
EP 1 688 398 describes similar K2O-containing lithium silicate glass ceramics which are moreover substantially free from ZnO. A heat treatment at 830 to 880° C. is applied to them to produce lithium disilicate.
U.S. Pat. No. 5,507,981 describes processes for producing dental restorations and glass ceramics that can be used in these processes. These are in particular lithium disilicate glass ceramics with a low level of Li2O which contain as a rule either Na2O or K2O.
U.S. Pat. No. 6,455,451 relates to lithium disilicate glass ceramics which contain further alkali metal oxides in addition to Li2O. However, the production of the desired lithium disilicate crystal phase requires high temperatures of from 800 to 1000° C.
WO 2008/106958 discloses lithium disilicate glass ceramics for veneering zirconium oxide ceramics. The glass ceramics contain Na2O and are produced by heat treatment of nuclei-containing glasses at 800 to 940° C.
WO 2009/126317 describes GeO2-containing lithium metasilicate glass ceramics which also contain K2O. The glass ceramics are processed to form dental products primarily by machining.
WO 2011/076422 relates to lithium disilicate glass ceramics which also contain K2O in addition to high levels of ZrO2 or HfO2. The crystallization of lithium disilicate takes place at high temperatures of from 800 to 1040° C.
Common to the known lithium disilicate glass ceramics is that they require heat treatments at more than 800° C. in order to effect the precipitation of lithium disilicate as main crystal phase. A large quantity of energy is therefore also necessary for their preparation. Further, with the known glass ceramics the alkali metal oxides, such as in particular K2O or Na2O, are as a rule present as essential components which are apparently required for the production of glass ceramics with the desired properties and in particular the formation of the sought lithium disilicate main crystal phase.